This invention relates to rotational deflection measurement devices, and in particular to such devices employing intra-cavity laser modulators.
The oscillation frequencies of the various modes of a laser beam within a laser cavity can be modulated by the four fundamental optical effects which are: (1) the optical activity effect, (2) the Faraday effect, (3) the Fresnel-Fizeau effect, and (4) the birefringence effect. When a light transmissive or partially transmissive element is placed within a laser cavity, the element may be utilized to have a desired influence employing one or more of the above listed effects. At least two of the effects are needed to produce quadrature mode operation.
For example, in U.S. Pat. No. 3,506,362 issued to W. M. Doyle, et al on Apr. 14, 1970, there is disclosed the use of a single effect, that of a rotatable optical transmissive birefringent device within a laser cavity with its optical axis being parallel to the axis of rotation and perpendicular to the direction of the laser beam. With the Doyle arrangement, the laser oscillations are limited to linear polarization modes of excitation of the optical cavity and the effects measured by a rotation of the transmissive device are limited to those based on birefringence only. The Doyle arrangement provides for a natural birefringent splitting of the optical frequency modes present and the production of a beat frequency of the order of 10.sup.8 Hertz for zero deflection or rotation of the optically transmissive device used. Hence, accurate detection of a rotation as described in the Doyle patent requires a frequency stability of the optically transmissive device (as well as the laser) to one part in 10.sup.8. Such stability is extremely difficult if not impossible to achieve in view of ever present thermal gradients, stresses, electromagnetic effects, etc., which will all adversely affect the beat frequency stability. The Doyle arrangement is not a null measuring technique, but rather is used to compare two non-degenerate large numbers by subtracting the numbers to obtain a small difference--a technique well known in science to be fraught with error and noise problems. A null comparison device is much preferred since it measures the desired small quantity directly.
Another example of intra-cavity laser modulation is contained in U.S. Pat. No. 3,786,681 issued to Kiehn on Jan. 22, 1974. That patent discloses the use of a stress-birefringent optically active element placed in the path of a laser beam with the element's optical axis being parallel to the direction of the laser beam. The cylindrically shaped element is fixedly mounted at one end and adapted with a lever arm at the other end for applying a torque on the element. The resulting stress-birefringent optical activity effect causes the production of a frequency difference between a plurality of circular polarization modes. The torque applied is around the optical axis of the crystal. Additionally, the Kiehn patent discloses the use of a ring laser system employing more modes of operation than possible with the system of Doyle. When the element of the Kiehn patent is torqued about its optical axis, it causes variations in each of the modes of operation of the laser beam. By connecting suitable torquing mechanisms to the element, it is possible to produce frequency shifts in the laser operating modes which are directly proportional to gravity variations, pressure variations, accelerations, or other desired physical measurements.
The techniques disclosed in Doyle and Kiehn are very useful and suitable for some purposes, but they by no means exhaust the possibilities for accurate measurements made with intra-cavity laser modulators. In particular, the instant application discloses a very useful alternative technique for making sensitive measurements with laser beam modulators, and utilizes the fact that certain combinations of the effects described above will produce quadrature modes of operation in a ring laser.